The present invention relates to an electrophotosensitive material.
As an electrophotosensitive material for use in image forming apparatuses such as electrostatic copiers, laser beam printers, plain paper facsimiles and the like, a so-called organic electrophotosensitive material is widespread which comprises a combination of the following components:
a charge generating material for generating an electric charge (positive hole and electron) when exposed to light;
a charge transport material for transporting the generated electric charge; and
a binder resin.
The charge transport materials fall into two broad categories which include a positive-hole transport material for transporting positive holes of the electric charge, and an electron transport material for transporting electrons.
The organic electrophotosensitive material has an advantage over an inorganic electrophotosensitive material employing an inorganic semiconductor material in that the organic electrophotosensitive material is fabricated more easily at less production costs than the latter.
In addition, the organic electrophotosensitive material also has a merit of greater freedom of function design by virtue of a wide variety of options for materials including charge generating materials, charge transport materials, binder resins and the like.
The organic electrophotosensitive material is constructed by forming a single-layer or multi-layer photosensitive layer over a conductive substrate.
The single-layer photosensitive layer is formed by dispersing a charge generating material and a charge transport material (a positive-hole transport material and/or an electron transport material) in a binder resin.
The multi-layer photosensitive layer is formed by forming a lamination of the charge generating layer containing the charge generating material and the charge transport layer containing the charge transport material (the positive-hole transport material or the electron transport material).
Despite the aforementioned various merits, the organic electrophotosensitive material is susceptible to scratches, mars and the like in an actual use environment, thus suffering a smaller durability than the inorganic electrophotosensitive material.
With an aim at increasing the durability of the organic electrophotosensitive material by solving the above problem, study has been made on an approach to overlay a surface protective layer on an outermost layer.
The widely used surface protective layer is exemplified by an organic layer which is preferable in the light of adhesion to and affinity with the organic photosensitive layer, integrity as a lamination, and consistency in the film forming process. A usable surface protective layer includes, for example, a layer of binder resin, and a layer of binder resin having conductive particles, such as of metal oxides, dispersed therein.
However, the electrophotosensitive material employing such an organic layer as the surface protective layer suffers the drawbacks of an increased residual potential and a lowered chargeability when repeatedly used for image forming processes, and of significant variations in the photosensitivity characteristics due to environmental changes (temperature, humidity and the like).
In this connection, more recent years have seen investigations made on the use of an inorganic layer as the surface protective layer, the inorganic layer comprising an inorganic material such as metallic elements, carbon and inorganic compounds containing any of these elements, and having high hardness and wear resistance. The inorganic surface protective layer may be laid over the organic photosensitive layer by, for example, the vapor deposition methods such as sputtering, plasma CVD, photo CVD or the like.
The inorganic surface protective layer is employed for the purposes of protecting the organic photosensitive layer and overcoming the above problem. Specifically, the electrophotosensitive material with the inorganic surface protective layer laid over the organic photosensitive layer has functions associated with the characteristics of the individual layers thereof, the organic photosensitive layer involved in the generation and transport of the electric charge, the surface protective layer responsible for ensuring the good durability and environmental resistance.
As compared with the organic surface protective layer, however, the inorganic surface protective layer has a lower ability to achieve a sufficient adhesion to the organic photosensitive layer. Even if adjustments for the deposition process or the deposition conditions may provide the inorganic layer with a sufficient initial adhesion to the organic layer, the inorganic layer is prone to suffer cracks or delamination due to various stresses imposed thereon under the actual use environment or during the long-term storage thereof.
In the combination of the organic photosensitive layer and the inorganic surface protective layer, which are formed of different materials, there are not attained as good adhering relation, affinity and integrity as in the combination of the organic layers or of the inorganic layers. That is, the organic layer and the inorganic layer are often merely combined with each other through a very small binding strength.
Accordingly, when subjected to mechanical stresses such as of contact pressure from a cleaning blade of the image forming apparatus, or thermal stresses due to repeated cycles of heating during the operation of the apparatus and cooling during the nonoperation thereof, or temperature changes during storage, the electrophotosensitive material will suffer cracks in the inorganic surface protective layer or delamination of the surface protective layer from the organic photosensitive layer as a result of increased differences between the hardnesses, flexibilities, expansion/shrinkage properties or the like of these layers.
In the present conditions, therefore, the conventional inorganic surface protective layer is yet to be put to practical use because it has not achieved a sufficient effect to increase the durability of the organic photosensitive layer.
It is an object of the invention to provide an organic electrophotosensitive material comprising an inorganic surface protective layer less prone to suffer cracks or delamination and excellent in physical stability, thereby achieving a greater durability as compared with the prior-art products.
For achieving the above object, the inventors have analyzed and investigated the film forming process for the inorganic surface protective layer.
As a result, the inventors have discovered that a condition of the surface protective layer initially deposited on the outermost part of the organic photosensitive layer has a significant influence on the physical stability of the surface protective layer subsequently deposited.
At an initial stage of the film formation, the inorganic material forming the surface protective layer is somehow combined with a part of the material of the organic photosensitive layer that is exposed at the outermost part thereof, thereby forming a nucleus for film growth. A film of the inorganic material grows about the resultant nucleus and thus, the surface protective layer is formed. In the surface protective layer thus formed, the nucleus portion functions as a binding point with the organic photosensitive layer, ensuring the good adhesion between these layers.
Therefore, the magnitude of binding strength between the organic photosensitive layer and the inorganic material at individual binding points as well as the per-area number of binding points namely the density of the binding points at an interface between the organic photosensitive layer and the surface protective layer give significant influences on the adhesion of the surface protective layer to the organic photosensitive layer and the physical stability of the surface protective layer.
Specifically, with increase in the binding strength between the organic photosensitive layer and the inorganic material and also in the density of the binding points at the interface between these layers, the surface protective layer is accordingly increased in the adhesion to the organic photosensitive layer, resulting in the greater physical stability.
As mentioned supra, the typical organic photosensitive layer has a structure wherein low molecular weight functional materials including the charge generating material, charge transport material and the like are dispersed in the binder resin forming the layer.
From the standpoint of the findings regarding the binding points, it is thought ideal that the binder resin, forming the layer and accounting for a major part thereof, act as the nucleus of film growth so as to be combined with the inorganic material forming the surface protective layer.
In the actual process, however, because of the stability and reactivity of the molecules per se or of the reaction site, the formation of the surface protective layer proceeds with some of the low molecular weight materials, that is exposed at the outermost part of the organic photosensitive layer, functioning as the nuclei of film growth, the low-molecular weight materials including the charge generating material, charge transport material and the like which are dispersed in the layer.
Hence, the properties of the low molecular weight materials, which include the reactivity and binding strength with the inorganic material, the degrees of the compatibility and affinity with the binder resin forming the organic photosensitive layer, the dimensions of the materials themselves (including not only the molecular weight but also the molecular or spatial extent), also significantly affect the adhesion to the organic photosensitive layer and the physical stability of the surface protective layer.
That is, as the low molecular-weight materials are increased in the reactivity and binding strength with the inorganic material, the surface protective layer is accordingly improved in the adhesion to the organic photosensitive layer and in the physical stability thereof.
Furthermore, as the low molecular weight materials are increased in the compatibility and affinity with the binder resin forming the organic photosensitive layer as well as in the dimensions thereof, a so-called anchor effect is accordingly increased so that the surface protective layer is also improved in the adhesion to the organic photosensitive layer and the physical stability thereof.
As to the combined form between the low molecular weight materials and the inorganic material, the most preferred is molecular bond in the light of the magnitude of the binding strength. However, if this bond should change the molecular structure to cause the production of an electric charge trap, the photosensitivity of the electrophotosensitive material might be decreased.
Therefore, an important consideration in the use of the low-molecular weight materials influence the need to prevent the reaction from transforming the molecular structure to a state reduced in the electrical properties.
Thus, the inventors have found that an electrophotosensitive material capable of forming preferable images cannot be obtained simply by overlaying on the conventional organic photosensitive layer a surface protective layer containing an inorganic material of a greater hardness.
Only after the fabrication of electrophotosensitive materials satisfying the various conditions described above, the inventors have finally discovered that the inorganic surface protective layer contributes to the improvement of the durability and environmental resistance of the electrophotosensitive material while maintaining the electrical characteristics of the organic photosensitive layer as they are.
Taking these findings into consideration, the inventors have made investigation into the various materials for forming the organic photosensitive layer. The invention has been achieved by the inventors"" study that a suitable material satisfying these requirements is a diphenylamine compound used as the positive-hole transport material and represented by the following formula (1): 
wherein xe2x80x98Axe2x80x99 denotes a group having at least one of aromatic groups, heterocyclic groups, double bond groups and conjugated double bond groups combined with two phenyl groups in the formula in a manner to jointly form a xcfx80-electron conjugated system, provided that when xe2x80x98Axe2x80x99 is the only one phenyl group that is directly combined with nitrogen atom in the formula, this phenyl group further possesses a group including one or more aromatic groups, heterocyclic groups, double bond groups or conjugated double bond groups which form the xcfx80-electron conjugated system jointly with these groups, or that when xe2x80x98Axe2x80x99 possesses a double bond group directly combined with nitrogen atom in the formula and one phenyl group attached to its end, this phenyl group further possesses a group including one or more aromatic groups, heterocyclic groups, double bond groups or conjugated double bond groups which form the xcfx80-electron conjugated system jointly with these groups; R1 and R2 are the same or different and each denote a hydrogen atom, alkyl group, alkoxy group, aralkyl group, aromatic group or halogen atom; R1 or R2 may form a condensed ring jointly with the phenyl group; and xe2x80x98axe2x80x99 and xe2x80x98bxe2x80x99 are the same or different and each denote an integer of 0 to 5.
In short, the electrophotosensitive material of the invention comprises the organic photosensitive layer and the inorganic surface protective layer laid over the conductive substrate in this order, wherein at least an outermost part of the organic photosensitive layer that contacts the surface protective layer contains the diphenylamine compound of the formula (1).
The diphenylamine compound of the formula (1) features a great reactivity with the inorganic material forming the surface protective layer because the xcfx80-electron conjugated system is spread across the molecules thereof so that the compound has a function to attract particularly a metallic element or carbon of the inorganic material at the initial stage of the film forming process.
Additionally, this function increases the ratio of the molecules of the diphenylamine compound exposed at the outermost part of the organic photosensitive layer that are combined with the inorganic material to form the nuclei of film growth. This results in a higher density of the binding points at the interface between these layers.
Furthermore, the higher the density of the binding points, the greater the film growth rate. Therefore, the time for film forming process may be reduced thereby minimizing damage on the organic photosensitive layer during the deposition of the surface protective layer by the vapor deposition methods or the like.
With a xcfx80-bond of the double bond in the molecules split off, the diphenylamine compound is rigidly combined with a metallic element, carbon or the like via molecular bond.
In addition, the diphenylamine compound has a relatively greater molecular weight among the positive-hole transport materials. Because of the xcfx80-electron conjugated system spread across the molecules, the diphenylamine compound has a molecular structure spread in a plane-like fashion as a whole, thus having a great molecular or spatial extent. Furthermore, the compound is excellent in compatibility and affinity with the binder resin, presenting a good anchor effect on the binder resin.
Therefore, a great binding strength between the organic photosensitive layer and the inorganic material results.
According to the invention, the physical stability of the inorganic surface protective layer can be improved by increasing the adhesion thereof to the organic photosensitive layer. Thus, the inorganic protective layer is prevented from suffering the occurrence of cracks and delamination in the actual use environment or during the long-term storage. As a result, an electrophotosensitive material featuring a superior durability to the conventional ones is provided.
The diphenylamine compound has a great positive-hole transportability because of the xcfx80-electron conjugated system spread across the molecules thereof. Furthermore, the compound does not produce a deep electric charge trap even when the molecular structure thereof is changed by the molecular bond with a metal or carbon. In addition, the molecular bond occurs only in a small part of the diphenylamine compound that is exposed at the outermost part of the organic photosensitive layer, so that the most of the diphenylamine compound in the organic photosensitive layer maintains the excellent positive-hole transportability as it is. Hence, there is no fear of reduced photosensitivity of the electrophotosensitive material.
Besides the above merits, the diphenylamine compound is excellent in compatibility with the binder resin so that a large amount of diphenylamine compound may be uniformly dispersed in the binder resin without producing particle aggregation. As a result, the electrophotosensitive material of the invention also features good photosensitivity characteristics.
The invention will be described as below.
Diphenylamine Compound
In an electrophotosensitive material according to the invention, a diphenylamine compound contained in at least an outermost part of an organic photosensitive layer that is in contact with a surface protective layer is represented by the formula (1): 
wherein xe2x80x98Axe2x80x99 denotes a group having at least one of aromatic groups, heterocyclic groups, double bond groups and conjugated double bond groups combined with two phenyl groups in the formula in a manner to jointly form a xcfx80-electron conjugated system, provided that when xe2x80x98Axe2x80x99 is the only one phenyl group that is directly combined with nitrogen atom in the formula, this phenyl group further possesses a group including one or more aromatic groups, heterocyclic groups, double bond groups or conjugated double bond groups which form the xcfx80-electron conjugated system jointly with these groups, or that when xe2x80x98Axe2x80x99 possesses a double bond group directly combined with nitrogen atom in the formula and one phenyl group attached to its end, this phenyl group further possesses a group including one or more aromatic groups, heterocyclic groups, double bond groups or conjugated double bond groups which form the xcfx80-electron conjugated system jointly with these groups; R1 and R2 are the same or different and each denote a hydrogen atom, alkyl group, alkoxy group, aralkyl group, aromatic group or halogen atom; R1 or R2 may form a condensed ring jointly with the phenyl group; and xe2x80x98axe2x80x99 and xe2x80x98bxe2x80x99 are the same or different and each denote an integer of 0 to 5.
Examples of a suitable diphenylamine compound of the formula (1) includes compounds in which xe2x80x98Axe2x80x99 in the formula (1) is a group represented by a formula (A1):
xe2x80x94Ar1xe2x80x94(R3)cxe2x80x83xe2x80x83(A1)
wherein Ar1 denotes an aromatic group, heterocyclic group or group represented by a formula (A1a):
xe2x80x94Ar2xe2x80x94CHxe2x95x90CHxe2x80x94Ar3xe2x80x94CHxe2x95x90Exe2x80x94xe2x80x83xe2x80x83(A1a)
in which formula (A1a), Ar2 and Ar3 are the same or different and each denote an aromatic group, and xe2x80x98Exe2x80x99 denotes a nitrogen atom or a group represented by a formula (A1b): 
in which formula (A1b), Ar4 denotes an aromatic group; R3 denotes a hydrogen atom, aromatic group, heterocyclic group or group represented by a formula (A1c); 
provided that when Ar1 is an aromatic group derived from a benzene ring, R3 is not a hydrogen atom; xe2x80x98cxe2x80x99 is an integer of 1 or 2; Ar5 and Ar6 in the formula (A1c) are the same or different and each denote an aromatic group.
Examples of another suitable diphenylamine compound of the formula (1) include compounds in which xe2x80x98Axe2x80x99 in the formula (1) is a group represented by a formula (A2): 
wherein R4 denotes an aromatic group having 7 to 16 carbon atoms, heterocyclic group or group represented by a formula (A2a): 
in which formula (A2a), Ar7 denotes an aromatic group or two or more aromatic groups forming a xcfx80-electron conjugated system, Ar8 and Ar9 are the same or different and each denote an aromatic group; and xe2x80x98dxe2x80x99 denotes an integer of 0 or 1.
Examples of the aromatic group in the above formulas include groups derived from aromatic compounds such as benzene, toluene, xylene, biphenyl, o-terphenyl, m-terphenyl, p-terphenyl, naphthalene, anthracene, phenanthrene, pyrene, indene, azulene, heptalene, biphenylene, fluorene and the like.
Examples of the heterocyclic group include groups derived from heterocyclic compounds such as indole, quinoline, benzopyran, quinazoline, xanthene, carbazole, phenanthridine and the like.
Examples of the double bond group include xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94CHxe2x95x90Nxe2x80x94, xe2x80x94Nxe2x95x90Nxe2x80x94 and the like.
A specific example of the conjugated double bond group is exemplified by a group comprising two or more of one type or more than one types selected from the above double bond groups and combined in a manner to form the xcfx80-electron conjugated system.
These groups may optionally have a substituent. Examples of a suitable substituent include alkyl groups, alkoxy groups, aralkyl groups, aromatic groups, halogen atoms and the like.
Examples of the alkyl group equivalent to the above substituent or the groups R1, R2 include alkyl groups having 1 to 12 carbon atoms, such as methyl, ethyl, n-propyl(n-Pr), isopropyl(i-Pr), n-butyl(n-Bu), isobutyl(i-Bu), sec-butyl(s-Bu), tert-butyl(t-Bu), pentyl, isopentyl, neopentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl and the like.
Examples of the alkoxy group include alkoxy groups having 1 to 12 carbon atoms, such as methoxy, ethoxy, n-propoxy, isopropoxy, n-butoxy, isobutoxy, sec-butoxy, tert-butoxy, pentyloxy, isopentyloxy, neopentyloxy, hexyloxy, heptyloxy, octyloxy, nonyloxy, decyloxy, undecyloxy, dodecyloxy and the like.
Examples of the aralkyl group include aralkyl groups having 4 to 10 carbon atoms in its aryl potion, such as benzyl, benzhydryl, triphenylmethyl, phenethyl, thenyl, furfuryl and the like.
Examples of the aromatic group include the same groups as those mentioned supra.
Other usable substituents include, for example, hydroxyalkyl groups; alkoxyalkyl groups; monoalkyl aminoalkyl groups; dialkyl aminoalkyl groups; halogenated alkyl groups; alkoxycarbonylalkyl groups; carboxyalkyl groups; alkanoyloxyalkyl groups; aminoalkyl groups; amino group; hydroxy group; optionally esterified carboxyl groups; cyano group and the like.
Specific examples of the group represented by the formula (1) with xe2x80x98Axe2x80x99 denotes the formula (A1) include the following compounds.
Diphenylamine Compound (1-1) 
This compound is equivalent to a compound of the formula (1) wherein xe2x80x98Axe2x80x99 is represented by the formula (A1) in which Ar1 is represented by the formula (A1a), R3 is represented by the formula (A1c) and xe2x80x98cxe2x80x99 is 1; in which formula (A1a), xe2x80x98Exe2x80x99 is represented by the formula (A1b); wherein Ar2, Ar3 and Ar4 each denote a divalent aromatic group derived from a benzene ring; wherein Ar5 and Ar6 each denote a phenyl group. The groups R1, R2 and the symbols xe2x80x98axe2x80x99, xe2x80x98bxe2x80x99 are the same as defined in the foregoing. The groups R5 and R6 are the same or different and each denote a hydrogen atom, alkyl group, alkoxy group, aralkyl group, aromatic group or halogen atom. The symbols xe2x80x98exe2x80x99 and xe2x80x98fxe2x80x99 are the same or different and each denote an integer of 0 to 5. The groups R7 and R8 are the same or different and each denote a hydrogen atom or alkyl group.
The compound of the formula (1-1) is rigidly combined with a metal or carbon via molecular bond because the double bond in the molecule, particularly a xcfx80-bond of the xe2x80x94CHxe2x95x90CHxe2x80x94 bond is split off. Among the diphenylamine compounds of the formula (1), this compound has not only a greater molecular weight but also a greater molecular or spatial extent because of its molecular structure with seven benzene rings spread in a plane-like fashion. Therefore, the compound exhibits a particularly preferable anchor effect on the binder resin. Thus, the compound of the formula (1-1) further features a good effect to increase the binding strength between the organic photosensitive layer and the inorganic material in addition to the aforesaid effects.
The compound of the formula (1-1) is classified into three types of compounds respectively having two xe2x80x94CHxe2x95x90CHxe2x80x94 groups in an ortho(o-) position, a meta(m-) position, and a para(p-) position relative to the benzene ring in the molecular center. Any of these compounds are usable in the invention.
Specific examples of the compound of the formula (1-1) include compounds represented by formulas (1-1-1) to (1-1-31). 
This compound is equivalent to a compound of the formula (1) wherein xe2x80x98Axe2x80x99 is represented by the formula (A1) in which Ar1 is represented by the formula (A1a), R3 is represented by the formula (A1c), and xe2x80x98cxe2x80x99 is 1; in which formula (A1a), xe2x80x98Exe2x80x99 is represented by the formula (A1b), and Ar2 and Ar3 each denote a divalent aromatic group derived from a benzene ring. The groups R1, R2, Ar5 and Ar6 and the symbols xe2x80x98axe2x80x99 and xe2x80x98bxe2x80x99 are the same as defined in the foregoing. The groups R9 and R10 are the same or different and each denote a hydrogen atom, alkyl group, alkoxy group, aralkyl group, aromatic group or halogen atom. The symbols xe2x80x98gxe2x80x99 and xe2x80x98hxe2x80x99 are the same or different and each denote an integer of 0 to 4.
The compound of the formula (1-2) is rigidly combined with a metal or carbon via molecular bond because the double bond in the molecule, particularly the xcfx80-bond of the xe2x80x94CHxe2x95x90CHxe2x80x94 bond is split off. Among the diphenylamine compounds of the formula (1), this compound has not only a greater molecular weight but also a greater molecular or spatial extent because of its molecular structure with four benzene rings and two aromatic rings spread in a plane-like fashion. Therefore, the compound exhibits a particularly preferable anchor effect on the binder resin. Thus, the compound of the formula (1-2) further features a good effect to increase the binding strength between the organic photosensitive layer and the inorganic material in addition to the aforesaid effects.
Specific examples of the compound of the formula (1-2) include compounds represented by formulas (1-2-1) to (1-2-18). 
Diphenylamine Compound (1-3) 
This compound is equivalent to a compound of the formula (1) wherein xe2x80x98Axe2x80x99 is represented by the formula (A1) in which Ar1 is a divalent aromatic group derived from a benzene ring, R3 is represented by the formula (A1c), and xe2x80x98cxe2x80x99 is 1; and wherein Ar5 and Ar6 each denote a phenyl group. The groups R1, R2 and the symbols xe2x80x98axe2x80x99, xe2x80x98bxe2x80x99 are the same as defined in the foregoing. The groups R11, R12 and R13 are the same or different and each denote a hydrogen atom, alkyl group, alkoxy group, aralkyl group, aromatic group or halogen atom. The symbols xe2x80x98ixe2x80x99 and xe2x80x98jxe2x80x99 are the same or different and each denote an integer of 0 to 5, whereas xe2x80x98kxe2x80x99 denotes an integer of 0 to 4.
Among the diphenylamine compounds of the formula (1), the compound of the formula (1-3) is molecularly stronger and has a good stability against thermally or electrically accelerated atoms or ions, plasma, light rays, heat radiation and the like, and more particularly against plasma-excited light and activated ions. Therefore, the compound is unsusceptible to decomposition or change in properties when exposed to such an atmosphere during the deposition of the inorganic surface protective layer by the CVD method or the like. Thus, the compound of the formula (1-3) features, in addition to the aforesaid effects, a good effect to increase the binding strength between the organic photosensitive layer and the inorganic material and to maintain such a strength at high level.
Furthermore, the compound of the formula (1-3) is molecularly strong enough to be unsusceptible to decomposition or change in properties and hence is unlikely to produce the deep electric charge trap. Accordingly, the compound contributes to the improvement of the photosensitivity characteristics of the electrophotosensitive material.
The compound of the formula (1-3) is classified into three types of compounds respectively having two nitrogen atoms in an ortho(o-) position, a meta(m-) position, and a para(p-) position relative to the benzene ring in the molecular center. Any of these compounds are usable in the invention.
Specific examples of the compound of the formula (1-3) include compounds represented by formulas (1-3-1) to (1-3-27). 
Diphenylamine Compound (1-4) 
This compound is equivalent to a compound of the formula (1) wherein xe2x80x98Axe2x80x99 is represented by the formula (A1), in which Ar1 is a divalent aromatic group derived from a naphthalene ring, R3 is represented by the formula (A1c), and xe2x80x98cxe2x80x99 is 1; wherein Ar5 and Ar6 each denote a phenyl group. The groups R1, R2, R11 and R12 and the symbols xe2x80x98axe2x80x99, xe2x80x98bxe2x80x99, xe2x80x98ixe2x80x99 and xe2x80x98jxe2x80x99 are the same as defined in the foregoing. The group R14 denotes a hydrogen atom, alkyl group, alkoxy group, aralkyl group, aromatic group or halogen atom. The symbol xe2x80x981xe2x80x99 denotes an integer of 0 to 6.
Among the diphenylamine compounds of the formula (1), the compound of the formula (1-4) has not only a greater molecular weight but also a greater molecular or spatial extent because of its molecular structure with four benzene rings and one naphthalene ring spread in a plane-like fashion. Therefore, the compound exhibits a particularly preferable anchor effect on the binder resin. Thus, the compound of the formula (1-4) further features a good effect to increase the binding strength between the organic photosensitive layer and the inorganic material in addition to the aforesaid effects. The compound of the formula (1-4) include those with two nitrogen atoms attached to any positions of either one ring of the naphthalene ring at the center. Any of those compounds are usable in the invention. Examples of a usable compound include 2,3-compound with two nitrogen atoms attached to the 2- and 3-positions of its naphthalene ring; 1,2-compound with two nitrogen atoms attached to the 1- and 2-positions of its naphthalene ring; 1,3-compound with two nitrogen atoms attached to the 1- and 3-positions of its naphthalene ring; 1,4-compound with two nitrogen atoms attached to the 1- and 4-positions of its naphthalene ring, and the like.
Specific examples of the 2,3-compound include those represented by formulas (1-4a-1) to (1-4a-27). 
Specific examples of the 1,3-compound include those represented by formulas (1-4b-1) to (1-4b-26). 
Diphenylamine Compound (1-5) 
This compound is equivalent to a compound of the formula (1) wherein xe2x80x98Axe2x80x99 is represented by the formula (A1) in which Ar1 is a divalent aromatic group derived from biphenyl, R3 is represented by the formula (A1c), and xe2x80x98cxe2x80x99 is 1; wherein Ar5 and Ar6 each denote a phenyl group. The groups R1, R2, R11 and R12 and the symbols xe2x80x98axe2x80x99, xe2x80x98bxe2x80x99, xe2x80x98ixe2x80x99 and xe2x80x98jxe2x80x99 are the same as defined in the foregoing. The groups R15 and R16 are the same or different and each denote a hydrogen atom, alkyl group, alkoxy group, aralkyl group, aromatic group or halogen atom. The symbols xe2x80x98mxe2x80x99 and xe2x80x98nxe2x80x99 are the same or different and each denote an integer of 0 to 4.
Among the diphenylamine compounds of the formula (1), the compound of the formula (1-5) has not only a greater molecular weight but also a greater molecular or spatial extent because of its molecular structure with six benzene rings spread in a plane-like fashion. Therefore, the compound exhibits a particularly preferable anchor effect on the binder resin. Thus, the compound of the formula (1-5) further features a good effect to increase the binding strength between the organic photosensitive layer and the inorganic material, in addition to the aforesaid effects.
Specific examples of the compound of the formula (1-5) include those represented by formulas (1-5-1) to (1-5-21). 
Diphenylamine Compound (1-6) 
This compound is equivalent to a compound of the formula (1) wherein xe2x80x98Axe2x80x99 is represented by the formula (A1) in which Ar1 is a divalent aromatic group derived from biphenyl, R3 is represented by the formula (A1c), and xe2x80x98cxe2x80x99 is 1; wherein Ar5 and Ar6 each denote a phenyl group. The groups R1, R2, R11 and R12 and the symbols xe2x80x98axe2x80x99, xe2x80x98bxe2x80x99, xe2x80x98ixe2x80x99 and xe2x80x98jxe2x80x99 are the same as defined in the foregoing. The group R17 denotes a hydrogen atom, alkyl group, alkoxy group, aralkyl group, aromatic group or halogen atom. The symbol xe2x80x98oxe2x80x99 denotes an integer of 0 to 5.
Among the diphenylamine compounds of the formula (1), the compound of the formula (1-6) is molecularly stronger and has a good stability against thermally or electrically accelerated atoms or ions, plasma, light rays, heat radiation and the like, and more particularly against plasma-excited light and activated ions. Therefore, the compound is unsusceptible to decomposition or change in properties when exposed to such an atmosphere during the deposition of the inorganic surface protective layer by the CVD method or the like. Thus, the compound of the formula (1-6) features, in addition to the aforesaid effects, a good effect to increase the binding strength between the organic photosensitive layer and the inorganic material and to maintain such a strength at high level.
Furthermore, this compound has a function to increase the glass transition temperature of the photosensitive layer with the assistance of a biphenyl skeleton thereof thereby improving heat resistance of the photosensitive layer. This results in a further increase in the rigid bind as mentioned above and the effect of preventing cracks.
The compound of the formula (1-6) is classified into three types of compounds respectively having two nitrogen atoms in an ortho(o-) position, a meta(m-) position, and a para(p-) position relative to the benzene ring in the molecular center. Any of these compounds are usable in the invention.
Specific examples of the compound of the formula (1-6) include those represented by formulas (1-6-1) to (1-6-11). 
Diphenylamine Compound (1-7) 
This compound is equivalent to a compound of the formula (1) wherein xe2x80x98Axe2x80x99 is represented by the formula (A1) in which Ar1 is a divalent aromatic group derived from phenanthrene, R3 is represented by the formula (A1c), and xe2x80x98cxe2x80x99 is 1; wherein Ar5 and Ar6 each denote a phenyl group. The groups R1, R2, R11 and R12 and the symbols xe2x80x98axe2x80x99, xe2x80x98bxe2x80x99, xe2x80x98ixe2x80x99 and xe2x80x98jxe2x80x99 are the same as defined in the foregoing. The group R18 denotes a hydrogen atom, alkyl group, alkoxy group, aralkyl group, aromatic group or halogen atom. The symbol xe2x80x98pxe2x80x99 denotes an integer of 0 to 8.
Among the diphenylamine compounds of the formula (1), the compound of the formula (1-7) has not only a greater molecular weight but also a greater molecular or spatial extent because of its molecular structure with four benzene rings and one phenanthrene ring spread in a plane-like fashion. Therefore, the compound exhibits a particularly preferable anchor effect on the binder resin. Thus, the compound of the formula (1-7) further features a good effect to increase the binding strength between the organic photosensitive layer and the inorganic material, in addition to the aforesaid effects.
Specific examples of the compound of the formula (1-7) include those represented by formulas (1-7-1) to (1-7-26). 
Diphenylamine Compound (1-8) 
This compound is equivalent to a compound of the formula (1) wherein xe2x80x98Axe2x80x99 is represented by the formula (A1), in which Ar1 is a trivalent aromatic group derived from benzene ring, R3 is represented by the formula (A1c), and xe2x80x98cxe2x80x99 is 2; wherein Ar5 and Ar6 each denote a phenyl group. The groups R1, R2, R11 and R12 and the symbols xe2x80x98axe2x80x99, xe2x80x98bxe2x80x99, xe2x80x98ixe2x80x99 and xe2x80x98jxe2x80x99 are the same as defined in the foregoing. The two groups R11 and R12 or symbols xe2x80x98ixe2x80x99 and xe2x80x98jxe2x80x99 of the formula (A1c) may be the same or different.
Among the diphenylamine compounds of the formula (1), the compound of the formula (1-8) has not only a greater molecular weight but also a greater molecular or spatial extent because of its molecular structure with seven benzene rings spread in a plane-like fashion. Therefore, the compound exhibits a particularly preferable anchor effect on the binder resin. Thus, the compound of the formula (1-8) further features a good effect to increase the binding strength between the organic photosensitive layer and the inorganic material, in addition to the aforesaid effects.
Specific examples of the compound of the formula (1-8) include those represented by formulas (1-8-1) to (1-8-6). 
Diphenylamine Compound (1-9)
This compound is equivalent to a compound of the formula (1) wherein xe2x80x98Axe2x80x99 is represented by the formula (A1), in which Ar1 denotes an aromatic group or heterocyclic group, R3 denotes a hydrogen atom, aromatic group or heterocyclic group, and xe2x80x98cxe2x80x99 is 1; provided that when Ar1 is an aromatic group derived from benzene ring, R3 is not a hydrogen atom but either an aromatic group or a heterocyclic group; otherwise or when Ar1 is either an aromatic group or heterocyclic group having 7 or more carbon atoms, R3 is not limited to the above and may be a hydrogen atom, aromatic group or heterocyclic group. The groups R1, R2 and the symbols xe2x80x98axe2x80x99, xe2x80x98bxe2x80x99 are the same as defined in the foregoing.
Specific examples of the compound of the formula (1-9) include those represented by formulas (1-9-1) to (1-9-31). 
Diphenylamine Compound (1-10)
This compound is equivalent to a compound of the formula (1) wherein xe2x80x98Axe2x80x99 is represented by the formula (A2), in which R4 denotes an aromatic group, heterocyclic group, or group represented by the formula (A2a), in which formula (A2a), xe2x80x98dxe2x80x99 is 0. The groups R1, R2, Ar8 and Ar9 and the symbols xe2x80x98axe2x80x99 and xe2x80x98bxe2x80x99 are the same as defined in the foregoing.
Specific examples of the compound of the formula (1-10) include those represented by formulas (1-10-1) to (1-10-10). 
Diphenylamine Compound (1-11)
This compound is equivalent to a compound of the formula (1) wherein xe2x80x98Axe2x80x99 is represented by the formula (A2), in which R4 is a group represented by the formula (A2a), in which formula (A2a), xe2x80x98dxe2x80x99 is 1. The groups R1, R2, Ar7, Ar8 and Ar9 and the symbols xe2x80x98axe2x80x99, xe2x80x98bxe2x80x99 are the same as defined in the foregoing.
Specific examples of the compound of the formula (1-11) include those represented by formulas (1-11-1) to (1-11-23). 
The above diphenylamine compounds may be used alone or in combination with two or more types thereof.
Organic Photosensitive Layer
The organic photosensitive layer includes a single layer type and a multi-layer type, and the invention may be applicable to these types.
The single-layer photosensitive layer is formed by the steps of applying a coating solution to a conductive substrate and drying the solution, the coating solution prepared by dissolving or dispersing in a suitable organic solvent, any one of the diphenylamine compounds of the formula (1) serving as the positive-hole transport material, the charge generating material and the binder resin.
The single-layer photosensitive layer features a simple layer construction and good productivity.
The single-layer photosensitive layer may also contain the electron transport material. A photosensitive layer employing charge transport materials of opposite polarities is advantageous in that the single layer construction is positively and negatively chargeable.
The multi-layer photosensitive layer is formed by the steps of overlaying on the conductive substrate the charge generating layer containing the charge generating material, applying a coating solution containing the charge transport material and the binder resin onto the resultant charge generating layer, and drying the solution thereby forming the charge transport layer. Otherwise, the multi-layer photosensitive layer may also be obtained by forming the charge transport layer over the conductive substrate, followed by forming thereover the charge generating layer.
The charge generating layer may further contain a charge transport material of the opposite polarity to that of the charge transport layer.
There are a great variety of multi-layer photosensitive layers in correspondence to combinations of the orders of the formation of the charge generating layer and charge transport layer and the polarities of the charge transport materials contained in these layers.
It is to be noted in the invention that the upper layer defining the outermost part in contact with the surface protective layer must contain the diphenylamine compound of the formula (1) serving as the positive-hole transport material.
Accordingly, specific examples of the multi-layer photosensitive layer include the following two types:
(a) a negative-charge multi-layer photosensitive layer wherein the charge generating layer containing the charge generating material and, as required, the electron transport material is formed over the conductive substrate and then the charge transport layer containing the diphenylamine compound of the formula (1) is laid over the charge generating layer; and
(b) a negative-charge multi-layer photosensitive layer wherein the charge transport layer containing the electron transport material is formed over the conductive substrate, and then the charge generating layer containing the charge generating material and the diphenylamine compound of the formula (1) is laid over the charge transport layer.
It is noted that the charge generating layer has quite a small thickness as compared with the charge transport layer and hence, the construction (a) with the charge transport layer laid on the upper side is more preferred.
Examples of a usable charge generating material include powders of inorganic photoconductive materials such as selenium, selenium-tellurium, selenium-arsenic, cadmium sulfide, xcex1-silicon and the like;
a variety of known pigments including phthalocyanine pigments comprising crystalline phthalocyanine compounds of various crystalline forms such as metal-free phthalocyanine represented by a formula CG-1; 
xe2x80x83titanyl phthalocyanine represented by a formula CG-2; 
azo pigments, bisazo pigments, perylene pigments, anthanthrone pigments, indigo pigments, triphenylmethane pigments, threne pigments, toluidine pigments, pyrazoline pigments, quinacridone pigments, dithioketopyrolopyrrole pigments and the like.
The charge generating materials may be used alone or in combination of two or more types such that the photosensitive layer may have sensitivity at a desired wavelength range.
Particularly, a electrophotosensitive material having photosensitivity in the wavelength range of 700 nm or more is required by digital-optical image forming apparatuses such as laser beam printers, plain paper facsimiles and the like which utilize infrared light such as semiconductor laser beam. Accordingly, phthalocyanine pigments out of the above exemplary compounds are preferably employed as the charge generating material.
Any of the various known electron-transporting compounds may be used as the electron transport material.
A preferred electron transport material include electron-attracting compounds which include, for example, benzoquinone compounds, diphenoquinone compounds such as 2,6-dimethyl-2xe2x80x2,6xe2x80x2-t-butylbenzoquinone represented by a formula (ET-1); 
naphthoquinone compounds, malononitrile, thiopyran compounds, tetracyanoethylene, 2,4,8-trinitrilothioxanthone, fluorenone compounds such as 2,4,7-trinitrilo-9-fluorenone, dinitrobenzene, dinitroanthracene, dinitroacridine, nitroanthraquinone, succinic anhydride, maleic anhydride, dibromomaleic anhydride, 2,4,7-trinitrofluorenoneimine compounds, ethylated nitrofluorenoneimine compounds, tryptanthrin compounds, tryptanthrinimine compounds, azafluorenone compounds, dinitropyridoquinazoline compounds, thioxanthene compounds, 2-phenyl-1,4-benzoquinone compounds, 2-phenyl-1,4-naphthoquinone compounds, 5,12-naphthacenequinone compounds, xcex1-cyanostilbene compounds, 4xe2x80x2-nitrostilbene compounds, salts formed by reaction between anionic radicals of benzoquinone compounds and cations.
These materials may be used alone or in combination of two or more types.
According to the invention, the diphenylamine compound of the formula (1), as the positive-hole transport material, may be used in combination with another positive-hole transport material.
Any of the various known positive-hole transporting compounds may be used as the additional positive-hole transport material.
Examples of a suitable positive-hole transport material include benzidine compounds, phenylenediamine compounds, naphthylenediamine compounds, phenantolylenediamine compounds, oxadiazole compounds such as 2,5-di(4-methylaminophenyl)-1,3,4-oxadiazole, styryl compounds such as 9-(4-diethylaminostyryl)anthracene, carbazole compounds such as poly-N-vinylcarbazole having a repeated unit represented by a formula (HT-1); 
organic polysilane compounds having a repeated unit represented by a formula (HT-2); 
wherein Ra and Rb are the same or different and each denote an alkyl group, alkoxy group, aryl group or aralkyl group, pyrazoline compounds such as 1-phenyl-3-(p-dimethylaminophenyl)pyrazoline, hydrazone compounds such as diethylaminobenzaldehyde diphenylhydrazone represented by a formula (HT-3); 
triphenylamine compounds such as tris(3-methylphenyl)amine represented by a formula (HT-4); 
indole compounds, oxazole compounds, isooxazole compounds, thiazole compounds, thiadiazole compounds, imidazole compounds, pyrazole compounds, triazole compounds, butadiene compounds, pyrene-hydrazone compounds, acrolein compounds, carbazole-hydrazone compounds, quinoline-hydrazone compounds, stilbene-hydrazone compounds, diphenylenediamine compounds and the like.
These compounds may be used alone or in combination of two or more types.
Examples of a usable binder resin include thermoplastic resins such as styrene polymers, styrene-butadiene copolymers, styrene-acrylonitrile copolymers, styrene-maleic acid copolymers, acrylic polymers, styrene-acryl copolymers, polyethylene, ethylene-vinyl acetate copolymers, chlorinated polyethylene, polyvinyl chloride, polypropylene, copolymers of vinyl chloride and vinyl acetate, polyester, alkyd resins, polyamide, polyurethane, polycarbonate, polyarylate, polysulfone, diarylphthalate resins, ketone resins, polyvinylbutyral resins, polyether resins and the like;
crosslinking thermosetting resins such as silicone resins, epoxy resins, phenol resins, urea resins, melamine resins and the like; and
photosetting resins such as epoxy-acrylate, urethane-acrylate and the like.
These resins may be used alone or in combination of two or more types.
Where a high-molecular positive-hole transport material such as poly-N-vinylcarbazole or the organic polysilane compound described above is used in combination with the diphenylamine compound of the formula (1), the aforesaid binder resin may be dispensed with because the former compound serves as the binder resin, as well.
Additionally to the above components, the photosensitive layer may further contain any of the various additives such as fluorene, ultraviolet absorber, plasticizer, surfactant, leveling agent and the like. For an increased photosensitivity of the electrophotosensitive material, there may be further added a sensitizer such as terphenyl, halonaphthoquinone, acenaphthylene or the like.
The single-layer photosensitive layer may preferably contain 0.1 to 50 parts by weight or particularly 0.5 to 30 parts by weight of charge generating material, and 5 to 500 parts by weight or particularly 25 to 200 parts by weight of positive-hole transport material, based on 100 parts by weight of binder resin.
Where the diphenylamine compound of the formula (1) is used alone, the mixing ratio of the positive-hole transport material means that of the diphenylamine compound. Where the diphenylamine compound is used in combination with another positive-hole transport material, the mixing ratio of the positive-hole transport material means the combined ratio of the diphenylamine compound and the additional positive-hole transport material.
Where the diphenylamine compound is used in combination with another positive-hole transport material, the additional positive-hole transport material may preferably be present in such a small amount that the aforesaid effect of the diphenylamine compound may not be decreased. More specifically, the additional positive-hole transport material may be present in concentrations of not more than 30 parts by weight based on 100 parts by weight of diphenylamine compound.
Where the electron-transport material is used in combination with the diphenylamine compound, the electron-transport material may preferably be present in concentrations of 5 to 100 parts by weight or particularly 10 to 80 parts by weight based on 100 parts by weight of binder resin. In this case, the total amount of the positive-hole transport material and the electron-transport material may preferably be in the range of 20 to 500 parts by weight or particularly 30 to 200 parts by weight based on 100 parts by weight of binder resin.
The single-layer photosensitive layer may preferably have a thickness of 5 to 100 xcexcm or particularly 10 to 50 xcexcm.
The charge generating layer of the multi-layer photosensitive layer may be formed from the charge generating material alone or formed from the binder resin in which the charge generating material and, as required, the electron transport material are dispersed. In the latter case, it is preferred to employ 5 to 1,000 parts by weight or particularly 30 to 500 parts by weight of charge generating material and 1 to 200 parts by weight or particularly 5 to 100 parts by weight of electron transport material based on 100 parts by weight of binder resin.
The charge transport layer may preferably contain the positive-hole transport material in concentrations of 10 to 500 parts by weight or particularly 15 to 200 parts by weight based on 100 parts by weight of binder resin.
Similarly to the single-layer photosensitive layer, the mixing ratio of the positive-hole transport material means that of the diphenylamine compound of the formula (1) when the diphenylamine compound is used alone. Where the diphenylamine compound is used in combination with another positive-hole transport material, the mixing ratio of the positive-hole transport material means the combined ratio of the diphenylamine compound and the additional positive-hole transport material.
Where the diphenylamine compound is used in combination with another positive-hole transport material, the additional positive-hole transport material may preferably be present in such a small amount that the aforesaid effect of the diphenylamine compound may not be decreased. More specifically, the additional positive-hole transport material may be present in concentrations of not more than 30 parts by weight based on 100 parts by weight of diphenylamine compound.
As to the thickness of the multi-layer photosensitive layer, the charge generating layer may preferably have a thickness of 0.01 to 5 xcexcm or particularly 0.1 to 3 xcexcm, whereas the charge transport layer may preferably have a thickness of 2 to 100 xcexcm or particularly 5 to 50 xcexcm.
An intermediate layer or barrier layer may be formed between the organic photosensitive layer of the single-layer type or multi-layer type and the conductive substrate or between the charge generating layer and the charge transport layer of the multi-layer photosensitive layer, so long as such a layer does not decrease the effect of the characteristics of the electrophotosensitive material.
Where each layer forming the electrophotosensitive material is formed by the coating method, the charge generating material, charge transport material, and binder resin may be dispersed, by mixing, into an organic solvent using a roll mill, ball mill, attritor, paint shaker, ultrasonic disperser or the like, thereby to prepare a coating solution, which may be applied and dried by the known means.
Examples of a usable organic solvent include alcohols such as methanol, ethanol, isopropanol, butanol and the like;
aliphatic hydrocarbons such as n-hexane, octane, cyclohexane and the like;
aromatic hydrocarbons such as benzene, toluene, xylene and the like;
halogenated hydrocarbons such as dichloromethane, dichloroethane, carbon tetrachloride, chlorobenzene and the like;
ethers such as dimethyl ether, diethyl ether, tetrahydrofuran, 1,4-dioxane, ethyleneglycol dimethyl ether, diethyleneglycol dimethyl ether and the like;
ketones such as acetone, methyl ethyl ketone, cyclohexanone and the like;
esters such as ethyl acetate, methyl acetate and the like; and
dimethylformaldehyde, dimethylformamide, dimethyl sulfoxide and the like. These solvents may be used alone or in combination of two or more types.
The coating solution may further contain a surfactant, leveling agent or the like for increasing the dispersibility of the charge generating material and charge transport material, and the surface smoothness of the photosensitive layer.
Surface Protective Layer
The inorganic surface protective layer is exemplified by a variety of surface protective layers comprising at least one element selected from the group consisting of metallic elements (the elements on the left side of a line interconnecting boron (B) and astatine (At) in the long-form periodic table) and carbon, or an inorganic compound containing any of these elements.
The surface protective layer may be formed by any of the various known vapor deposition methods including the chemical vapor deposition methods such as plasma CVD, photo CVD and the like, and the physical vapor deposition methods such as sputtering, vacuum deposition, ion plating and the like.
In the chemical vapor deposition method such as plasma CVD, there are formed:
1. a film comprising carbon (C) and/or silicon (Si) of the 14-group elements, that is, carbon (C) film, silicon (Si) film and silicon-carbon (Sixe2x80x94C) composite film;
2. a film comprising a compound containing the aforesaid carbon (C) and/or silicon (Si) and at least one element selected from the group consisting of boron (B) and aluminum (Al) of the 13-group elements; nitrogen (Ni) and phosphorus (P) of the 15-group elements; oxygen (O) and sulfur (S) of the 16-group elements; fluorine (F), chlorine (Cl) and bromine (Br) of the 17-group elements; the film including, for example, silicon-nitrogen (SiN) composite film, silicon-oxygen (SiO) composite film, carbon-fluorine (CF) composite film, carbon-nitrogen (CN) composite film, carbon-boron (CB) composite film, carbon-oxygen (CO) composite film and the like; and
3. a film comprising a compound containing boron (B) and/or aluminum (Al) of the 13-group elements and at least one selected from the group consisting of the aforesaid elements including nitrogen (N), phosphorus (P), oxygen (O), sulfur (S), fluorine (F), chlorine (Cl) and bromine (Br), the film including, for example, boron-nitrogen (BN) composite film, aluminum-nitrogen (AlN) composite film and the like.
These films may contain a fractional amount of hydrogen (H) for an improved electrical characteristics of the surface protective layer.
In the chemical vapor deposition method, a usable raw material gas for introduction of a constituent element of the surface protective layer include the molecules of the constituent elements, and compounds thereof such as oxides, hydrides, nitrides and halides thereof, the compounds capable of presenting a gaseous state under normal temperature and pressure conditions or of being readily gassified under film forming conditions. As required, these compounds may be diluted with a gas such as hydrogen gas (H2), helium gas, argon gas, neon gas or the like.
Specific examples of the raw material gas include:
silane gas (SiH4) and disilane gas (Si2H6) for silicon introduction;
methane gas (CH4), ethane gas (C2H6), propane gas (C3H8) and ethylene gas (C2H4) for carbon introduction;
fluorine gas (F2), bromine monofluoride gas (BrF), chlorine difluoride gas (ClF2), carbon tetrafluoride gas (CF4) and silicon tetrafluoride gas (SiF4) for fluorine introduction;
nitrogengas (N2), ammoniagas (NH3), nitrogenoxide gas (NOx) for nitrogen introduction;
and boron hydride gas such as diborane gas (B2H6), and tetraborane gas (B4H10) for boron introduction; and the like.
Similarly, the introduction of the other constituent elements may employ compounds capable of presenting a gaseous state under normal temperature and pressure conditions or of being readily gassified under film forming conditions.
In the physical vapor deposition method, or particularly in the sputtering or ion plating method, there are formed films, besides the aforesaid films, which each comprise one or more than one metallic elements selected from the group consisting of, for example, gallium (Ga), indium (In) and the like of the 13-group elements; germanium (Ge), tin (Sn), lead (Pb) and the like of the 14-group elements; arsenic (As), antimony (Sb) and the like of the 15-group elements; selenium (Se) and the like of the 16-group elements, or which each comprise an inorganic compound comprising any of the above elements.
Preferred as the inorganic surface protective layer are, for example, the carbon (C) film, silicon-carbon (SiC) composite film and the like.
The thickness of the inorganic surface protective layer may preferably be in the range of 0.01 to 30 xcexcm or particularly of 0.1 to 10 xcexcm.
The inorganic film defining the surface protective layer may be in any of the amorphous form, microcrystalline form, and crystalline form. Further, the film may comprise a mixture of amorphous and crystalline particles.
Conductive Substrate
The conductive substrate may employ substrates formed from various materials having conductivity. Examples of a usable conductive substrate include those formed from metals such as iron, aluminum, copper, tin, platinum, silver, vanadium, molybdenum, chromium, cadmium, titanium, nickel, palladium, indium, stainless steel, brass and the like; those formed from a plastic material on which any of the above metals is deposited or laminated; and glass substrate coated with aluminum iodide, tin oxide, indium oxide or the like.
In short, the substrate itself may have the conductivity or the surface thereof may have the conductivity. It is preferred that the conductive substrate has a sufficient mechanical strength in use.
The conductive substrate may have any form, such as sheet, drum and the like, according to the construction of the image forming apparatus to which the conductive substrate is applied.